The need for cost-effective, high performance data storage has, for many applications, outpaced technology development. Enterprise storage, image archives, and entertainment content, among other applications, are driving the demand for enhanced data storage solutions. Several of these applications currently rely on storage technologies, such as optical, magneto-optical, and magnetic tape, that use removable media. These technologies, for the most part, have relatively limited improvements remaining on their roadmaps for attaining increased data density, or have limitations in achievable data rates, or in random access. Holographic data storage (HDS), on the other hand, promises both near-term performance comparable to the most optimistic long-term projections for these technologies, and a technology roadmap with many years of rapidly increasing data storage density and data transfer rate in combination with random access.
A practical embodiment of an HDS system uses relatively thin recording material, such as photopolymers, in combination with, for example, a 4f optical imaging system. Mutually coherent signal and reference beams form an interference pattern in the volume of their overlap. A hologram is recorded when light-induced changes in the storage medium, such as photopolymerization, produce a record of the resulting interference pattern. Reconstruction of the recorded hologram is accomplished by firstly illuminating the hologram with a reference beam and secondly imaging the diffracted light onto the detector array.
Recording many independent holograms in the same volume of material enhances data density. This process, called multiplexing, requires that each multiplexed hologram be recorded with a unique reference beam. Many multiplexing procedures have been described in the literature (see for example G. Barbastathis and D. Psaltis, “Volume Holographic Multiplexing Methods”, Holographic Data Storage, H. J. Coufal, D. Psaltis, and G. T. Sincerbox (Eds.), Springer-Verlag, 2000). A particularly useful multiplexing procedure for relatively thin recording material uses a collimated reference beam, and combines angular and peristrophic (azimuthal) multiplexing techniques [see D. A. Waldman, H.-Y. S. Li, and E. A. Cetin, “Holographic Recording Properties in Thick Films of ULSH-500 Photopolymer”, Proceedings of SPIE, Vol. 3291, pp. 89-103 (1998) and A. Pu and D. Psaltis, “High-density recording in photopolymer-based holographic three-dimensional disks”, Appl. Optics, Vol. 35, No. 14, pp 2389-2398 (1996).
HDS systems that operate to maximize the data density, for a recording material of a particular thickness, use the highest numerical aperture (NA) lenses for the Fourier transform lens pair that said 4f optical imaging system can accommodate. Unfortunately, the use of high NA (NA ≧0.2 for HDS systems) lenses, such as in the conventional 4f optical system wherein the first and second Fourier transform lens are a matched pair and thus have identical values of NA, can introduce several factors that contribute to the substantial decreases of signal-to-noise (SNR) in the HDS system. Most significantly, when high NA optics is used for the second Fourier transform lens, then substantially more scattered light is imaged to the detector plane than for lower NA optics. Light scattered from media or media substrates, along with light scattered from optical and mechanical surfaces is captured more efficiently by high NA optics due to the shorter working distance of said lenses and the larger acceptance field of the lens. The scattered light is imaged onto the pixilated detector and recognized as noise during hologram read-out. This phenomenon is especially evident in thin photopolymer-based media systems where a non-90 degree interbeam angle must be used for the recording geometry. The suppression of noise from various sources is critical to the maximization of storage densities, in particular the suppression of optical noise. A typical HDS system has several potential sources of optical noise including the aforementioned light scattered from the media and/or optical components, reflections from surfaces internal to the drive, and, additionally, image misalignment and distortion. In general, each of these potential noise sources become increasingly more evident and problematic in systems that endeavor to maximize areal density of stored data.
There is a need, therefore, for an apparatus and a method that improves areal data density while at the same time reducing optical noise at the detector plane that is due to scattered and stray light so as to achieve good SNR at high areal density.